Information Notice 2011-03, Nonconsecutive Criticality Safety Analyses for Fuel Storage

Nonconsecutive Criticality Safety Analyses for Fuel Storage

ML103090055
Person / Time
Issue date:	02/16/2011
From:	Blount T, John Tappert Office of New Reactors, Division of Policy and Rulemaking
To:
Purnell, B A, NRR/DPR, 415-1380
References
IN-11-003
Download: ML103090055 (5)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NEW REACTORS

WASHINGTON, DC 20555-0001 February 16, 2011 NRC INFORMATION NOTICE 2011-03: NONCONSERVATIVE CRITICALITY SAFETY

ANALYSES FOR FUEL STORAGE

ADDRESSEES

All holders of operating licenses or construction permits for a nuclear power reactor issued

under the provisions of Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic

Licensing of Production and Utilization Facilities.

All holders of or applicants for a standard design certification, standard design approval, manufacturing license, or combined license issued under 10 CFR Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform

addressees of possible nonconservative technical specifications related to the criticality safety

of fuel storage. The NRC expects that recipients will review the information for applicability to

their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this IN are not NRC requirements; therefore, no specific action or

written response is required.

BACKGROUND

Paragraph 50.68(b)(4) of 10 CFR 50.68, Criticality Accident Requirements, requires the

following:

If no credit for soluble boron is taken, the k-effective of the spent fuel storage

racks loaded with fuel of the maximum fuel assembly reactivity must not exceed

0.95, at a 95 percent probability, 95 percent confidence level, if flooded with

unborated water. If credit is taken for soluble boron, the k-effective of the spent

fuel storage racks loaded with fuel of the maximum fuel assembly reactivity must

not exceed 0.95, at a 95 percent probability, 95 percent confidence level, if

flooded with borated water, and the k-effective must remain below 1.0

(subcritical), at a 95 percent probability, 95 percent confidence level, if flooded

with unborated water.

NUREG/CR-6698, Guide for Validation of Nuclear Criticality Safety Calculational Methodology, January 2001 (Agencywide Document and Management System (ADAMS) Accession

No. ML050250061), provides guidance on determining the bias uncertainty for Monte Carlo

codes.

The primary NRC staff guidance regarding the depletion uncertainty is an internal NRC

memorandum from L. Kopp to T. Collins, Guidance on the Regulatory Requirements for

Criticality Analysis of Fuel Storage at Light-Water Reactor Power Plants, dated

August 19, 1998 (ADAMS Accession No. ML003728001) (Kopp Letter). The Kopp Letter is

referenced by virtually all spent fuel pool criticality license amendment requests submitted since

its issuance.

Regarding the depletion uncertainty, the Kopp Letter states the following:

A reactivity uncertainty due to uncertainty in the fuel depletion calculations should

be developed and combined with other calculational uncertainties. In the

absence of any other determination of the depletion uncertainty, an uncertainty

equal to 5 percent of the reactivity decrement to the burnup of interest is an

acceptable assumption.

DESCRIPTION OF CIRCUMSTANCES

Beaver Valley Power Station

By letter dated April 9, 2009, FirstEnergy Nuclear Operating Company submitted a license

amendment request for Beaver Valley Power Station, Unit 2 (BVPS-2), to modify the technical

specifications to support the installation of high-density storage racks in the BVPS-2 fuel pool.

A vendor provided a criticality safety analysis to support the BVPS-2 license amendment

request.

During the review of the BVPS-2 application, the NRC staff found that the statistical treatment of

the Monte Carlo code bias uncertainty used in the criticality safety analysis was inappropriately

based on the standard deviation of the mean k-effective value calculated for the critical

experiments. 10 CFR 50.68 requires that the estimated k-effective be less than 1.0 at a

95-percent probability, 95-percent confidence level (95/95 confidence). Therefore, the correct

statistical approach is to determine the Monte Carlo code bias uncertainty using the standard

deviation of the population about the mean, not the standard deviation of the mean. Using the

correct statistical approach, the bias uncertainty was found to be larger by 0.0079 over the

previous analysis. The large increase in the value for the bias uncertainty also resulted in a

significant increase in the estimated k-effective.

St. Lucie Nuclear Power Plant

By letter dated April 16, 2010, Florida Power and Light Company submitted information about

the current licensing basis for fuel storage criticality safety at St. Lucie Nuclear Power Plant, Unit 1, as part of a license amendment request for an extended power uprate. During the

acceptance review of the request, the NRC found that the current licensing basis criticality

safety analysis for fuel storage did not include the depletion uncertainty for the unborated cases.

10 CFR 50.68 requires licensees to demonstrate that k-effective is less than 1.0 with a

95/95 confidence for the unborated cases. When these demonstrations rely upon fuel depletion, the depletion uncertainty must be included in both the borated and unborated

analyses. The estimated k-effective will likely increase when the depletion uncertainty is

factored into the analysis.

DISCUSSION

The two issues discussed in this IN relate to the Monte Carlo code bias uncertainty and

the depletion uncertainty. Both of these uncertainties, if not properly treated, may lead

to nonconservative estimation of the maximum k-effective, and regulatory compliance

may not be assured. Analyses with small margins to the regulatory limit are especially

vulnerable to noncompliance with 10 CFR 50.68 and nonconservative technical

specifications if these issues are present.

In 10 CFR 50.68, the NRC requires a 95-percent probability with a 95-percent confidence level

that a calculation demonstrating subcriticality of a spent fuel storage rack actually is subcritical.

The primary NRC guidance for criticality code validation, NUREG/CR-6698, includes guidance

on the determination of the bias uncertainty for Monte Carlo codes based on the population of

critical experiments. Guidance for determining the bias uncertainty in NUREG/CR-6698 can be

used to ensure compliance with 10 CFR 50.68.

Most nuclear criticality safety analyses performed to support spent fuel pool licensing actions

include reactor depletion calculations, whether it is a boiling-water reactor establishing the most

reactive point in the life of a fuel assembly lattice or a pressurized-water reactor taking burnup

credit. Licensing actions that include reactor depletion calculations should consider the

uncertainty of those calculations to ensure the 95/95 confidence requirement for k-effective is

met. The Kopp Letter provides the NRCs current guidance for determining uncertainties for

reactor depletion calculations. As with other NRC guidance, addressees may choose an

alternative method for determining the depletion uncertainty, although additional NRC staff

review should be expected.

The NRC is working to improve its guidance in these areas. The NRC Office of Nuclear Reactor

Regulation (NRR) has established an Action Plan, On Site Spent Fuel Criticality Analyses

(ADAMS Accession No. ML101520463), to monitor and track these activities. The NRC has

received public comment on Draft Interim Staff Guidance DSS-ISG-2010-01, Staff Guidance

Regarding the Nuclear Criticality Safety Analysis of Spent Fuel Pools (ADAMS Accession

No. ML101520463), and expects to issue the final version in the first quarter of 2011. The NRC

expects to issue more durable guidance by the first quarter of 2013.

CONTACT

S

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contacts listed below or the appropriate NRR project manager.

/RA/ /RA/

John Tappert, Acting Director Thomas B. Blount, Acting Director

Division of Construction Inspection Division of Policy and Rulemaking

and Operational Programs Office of Nuclear Reactor Regulation

Office of New Reactors

Technical Contacts: Kent A. L. Wood, NRR Tony T. Nakanishi, NRR

301-415-4120 301-415-3211 E-mail: kent.wood@nrc.gov E-mail: tony.nakanishi@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

CONTACT

S

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contacts listed below or the appropriate NRR project manager.

/RA/ /RA/

John Tappert, Acting Director Thomas B. Blount, Acting Director

Division of Construction Inspection Division of Policy and Rulemaking

and Operational Programs Office of Nuclear Reactor Regulation

Office of New Reactors

Technical Contacts: Kent A. L. Wood, NRR Tony T. Nakanishi, NRR

301-415-4120 301-415-3211 E-mail: kent.wood@nrc.gov E-mail: tony.nakanishi@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

ADAMS Accession Number: ML103090055 TAC ME4423 OFFICE LPL2-2:DORL:NRR SRXB:DSS:NRR Tech Editor BC:SRXB:NRR D:DSS:NRR

NAME JPaige KWood KAzariah-Kribbs AUlses WRuland

DATE 11/05/2010 11/08/2010 11/10/ 2010 e-mail 01/24/2011 01/25/2011 OFFICE LA:PGCB:NRR PM:PGCB:NRR PM:PGCB:NRR BC:PGCB:NRR BC(A):DCIP:NRO

NAME CHawes BPurnell DBeaulieu SRosenberg JTappert

DATE 02/03/2011 02/09/2011 02/09/2011 02/10 /2011 02/11 /2011 OFFICE BC:SRSB:DSRA D:DSRA:NRO D(A):DPR:NRR

NAME JEDonoghue CEAder TBlount

DATE 02/01/2011 02/03/2011 02/16/2011 OFFICIAL RECORD COPY